It is well known that there is a need for maintaining the purity of various of the nitrogen oxides for critical uses in many high technology industries. Nitrogen forms a multiple of oxides which readily attack the iron-containing metals of the apparatus typically used in their manufacture, transport and/or storage, thus creating significant problems when the maintainence of a desired level of purity is required. For example, it is well known that need exists for the supply of highly pure grades of nitrogen tetroxide propellants for use as oxidizers for rocket engines during system design and development tests, at the launching sites of vehicles to be projected into space and in orbit, such as satellites, space shuttles, or the like. Propellant supply/oxidizer flow rate decay problems associated with impure propellant such as are prone to interfere with design/testing programs and launch scheduling of the vehicle, as well as its space flight life duration prospects have plagued the industry. For example, the publication "Flow Decay", pages 101, 103, 104, 115, 117, 122, 124, 126, 132, 133, 135, 136, and 139 of Final Scientific Report, June 30, 1972, AFRPL-TR-72-84 reports on various problems within the current state of the art.
Such problems are now of major concern due to the recent emergence of nitrogen tetroxide propellants as major oxidizers for liquid propellant rocket engines and occur in connection with the storing, transferring, testing, launching and maneuvering operations of satellites, MX missiles, space shuttles and the like. These problems are caused by the tendency of such oxidizers to corrode their storage, transport and transfer containers, as well as other iron-containing metal parts of the engine oxidizer supply system. Commercially available nitrogen tetroxide propellants are supplied to the test site or launch pad mounted vehicle, either into the vehicle inboard supply tanks or into close-by "ready storage" ground-based intermediary relay tanks. The tanks, piping, valves, etc., components furnished by manufacturers are typically made of stainless steel alloys and are invariably surface-coated by potentially contaminating materials such as metallic oxides and salts, as well as manufacturing residues such as oils, greases, grime, bits of metal and shop dirt. Unless such contaminants are initially completely removed from such components of the system, upon introduction of nitrogen tetroxide therein, undesired chemical reactions are fostered and contaminants are loosened and freed to enter the nitrogen tetroxide supply in suspension. Such materials in suspension tend to plug the filters and other components by way of example in a rocket engine propulsion system, such as the valves and injection orifices; thereby seriously interfering with the propellant transfer operations as well as the test or vehicle launching operations. Furthermore, and perhaps more importantly, the incidence thereof seriously reduces the reliable duration prospect of the rocket engine performance. Typical pre-storage and transfer/operational ambient temperature conditions contribute to these problems. Regardless of how pure the furnished nitrogen tetroxide may be, its introduction into an improperly prepared system will limit its usefulness.
Previously, various steps were taken to "clean" the components of such systems subsequent to their manufacture, employing well-known agents for removing the residues recognized to be potential contaminants and/or reactants for the nitrogen tetroxides to be handled. However, that state of the art did not qualify to satisfy the needs of present day technologies requiring the use of higher purity nitrogen tetroxide. The present invention is the result of the discovery that a novel combination of chemical agents, applied in a specific sequence, will prepare iron-containing metal components to perform with improved efficacy.
As a result of a professional search conducted with respect to this invention, applicant is aware of U.S. Pat. Nos. 3,522,093; 3,598,741; 3,553,016; 2,992,945; 3,401,061; 3,880,681; and 3,510,432. Each of these references is considered relevant to the subject of cleaning and/or passivating apparatus, but none teach the specific procedures disclosed and claimed herein.
One object of the present invention is to provide new and improved methods for chemically cleaning iron-containing metal surfaces of apparatus which may contact a nitrogen oxide.
Another object of the present invention is to provide a method for chemically cleaning stainless steel surfaces of apparatus used in the manufacture, transportation, storage and/or use of high purity nitrogen oxides.
A further object of the present invention is to provide chemically cleaned stainless steel apparatus which will markedly increase the shelf life of nitrogen oxides manufactured, transported and/or stored therein over that of similar uncleaned apparatus.
A still further object of the present invention is to provide a process for chemically cleaning chromium containing stainless steel apparatus used in the manufacture, transport, and/or storage of nitrogen tetroxide.
These and other objects of the present invention will become apparent to those skilled in the art from a consideration of the following specification and claims.